The invention pertains to a device for monitoring the state of a window pane, in particular, an automobile window pane, consisting of at least one optical transmitter which emits a light beam onto a pane, at least one optical receiver which receives the light of the light beam modulated by the pane and subsequently generates a reception signal, and an evaluation circuit that evaluates the reception signal to determine the state of the pane.
The comfort and safety of a motor vehicle can be enhanced by automatic operation of the windshield wiper system. Usually, optical rate sensors located directly on the inside of the pane are used for this purpose. The state of the window pane on the outside of the pane is detected by the sensor through the pane. The monitored region in this case is limited by the size of the sensor and is less than the field of view of the driver.
The present invention is based on the task of creating a device for monitoring the state of a window pane which can be flexibly adapted to different operating situations and which allows a dependable determination of the state of the window pane.
The task is achieved in that the transmitter and the receiver are located at a distance from the window pane.
Due to this distant placement of both the transmitter and the receiver from the pane, the advantage achieved is that the size of the monitored region can be selected independently of the dimensions of the device. An additional advantage consists in that the location of the monitored region can be largely user-selected and can be located, for example, in the immediate field of view of an automobile driver. Due to the potential for prior specification of a suitable size and location of the monitored region, the dependability of the attainable evaluation result can be improved significantly.
Even though the measurement can also theoretically take place during transmission, the receiver preferably detects the light of the transmitter through reflection or scattering off the pane. A sufficiently large monitored region with simultaneously good intensity of the reflected light can be attained when the transmitter and the receiver are located at a distance of about 10 to 30 cm from the pane.
Preferably the light spot projected onto the pane by the light beam has a surface area of at least 25 cm2 and preferably about 100 cm2. The general state of the pane will then be represented in sufficient measure by the reflected light, and local changes in the state of the pane due to, for example, an area of dirt or a single, large rain drop, are not problems because of the size of the monitored region and they cannot falsify the monitored result, as is the case for sensors which are located directly on the glass and thus necessarily have a small monitored region.
The value of the monitored result will be increased when the state of the window pane that is being monitored is in the region of the immediate field of view of a driver.
Preferably, the transmitter operates in the infrared range, because this will prevent a driver or another person that is looking through the pane from being adversely affected by interfering reflections. Furthermore, it is an advantage that low-cost optical-electronic components operating in the infrared range, such as Si-photodiodes (as the receiver) and infrared LEDs (as the transmitter) are available.
According to one preferred design format of the present invention, the device is composed of a number of optical transmitters. Due to several optical transmitters, a greater transmission power can be achieved, so that the distance between the sensor and the pane can be increased. This makes possible more favorable monitoring geometries and, in addition, allows the angle between the optical axis of the transmitter and receiver, respectively, and the pane to be selected as more flat.
Even though basically several optical receivers can be provided, one preferred embodiment is characterized in that the device is composed of only one optical receiver.
In the case of several optical transmitters and one single receiver, one favorable embodiment of the invented evaluation circuit is characterized in that the circuit is composed of a discriminator stage which uses the reception signal to derive a first and a second reception signal according to the acquired modulated light from one or more first and one or more second optical transmitters. Due to this property, a transmitter-specific and, thus, also light-spot-specific, evaluation of the reception signal provided by the single receiver will be possible, which in practice allows a simultaneous monitoring of different regions of the window pane.
One simple possibility for forming the first and the second reception signals consists in that the first and second optical transmitter is controlled by a pulse signal of different phase and the discriminator stage is composed of a phase-synchronous demodulator.
Preferably, the evaluation circuit is composed of a difference stage which forms a difference signal from the first and the second reception signals. Different states of the window pane can be recognized from the temporal change in behavior of the difference signal. Whereas adhered dirt or even damage to the window pane will cause sudden, static changes, fast changes are caused by large raindrops and slow changes by smaller raindrops. Based on the state of the window pane detected in this manner, additional measures can be undertaken, such as, for example, the regulation of the speed of the windshield wipers.
A particularly compact design of the invented device is attained when the transmitter and the receiver are located in a common module or housing.
According to one preferred configuration of the present invention, on the input side of the optical receiver there is a reception lens which concentrates the light modulated by the pane onto the receiver. Due to this property, the sensitivity of the reception device will be increased and thus a greater distance between the sensor and the pane will be possible.
Due to the reception lens, a reception zone will be defined on the window pane. With regard to attaining the greatest measurement sensitivity, it is preferable to design the reception lens so that a reception zone defined on the pane by the reception lens matches the illuminated zone formed on the pane by the light spots of the light beam.
Due to the optical filter located on the input side in front of the optical receiver, interference caused by incident, secondary light can be prevented as long as the secondary light has a different wavelength than the transmitted light. In the case of a sensor operating in the infrared range, visible, scattered light can thus be effectively excluded as a potential source of interference.